Re: [PATCH v2] locking/local_lock, mm: Replace localtry_ helpers with local_trylock_t type

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Apr 2, 2025 at 1:56 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Tue, Apr 01, 2025 at 01:52:45PM -0700, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@kernel.org>
> >
> > Partially revert commit 0aaddfb06882 ("locking/local_lock: Introduce localtry_lock_t").
> > Remove localtry_*() helpers, since localtry_lock() name might
> > be misinterpreted as "try lock".
> >
> > Introduce local_trylock[_irqsave]() helpers that only work
> > with newly introduced local_trylock_t type.
> > Note that attempt to use local_trylock[_irqsave]() with local_lock_t
> > will cause compilation failure.
> >
> > Usage and behavior in !PREEMPT_RT:
> >
> > local_lock_t lock;                     // sizeof(lock) == 0
> > local_lock(&lock);                     // preempt disable
> > local_lock_irqsave(&lock, ...);        // irq save
> > if (local_trylock_irqsave(&lock, ...)) // compilation error
> >
> > local_trylock_t lock;                  // sizeof(lock) == 4
>
> Is there a reason for this 'acquired' to be int? Can it be uint8_t? No
> need to change anything here but I plan to change it later to compact as
> much as possible within one (or two) cachline for memcg stocks.

I don't see any issue. I can make it u8 right away.

> > local_lock(&lock);                     // preempt disable, acquired = 1
> > local_lock_irqsave(&lock, ...);        // irq save, acquired = 1
> > if (local_trylock(&lock))              // if (!acquired) preempt disable
> > if (local_trylock_irqsave(&lock, ...)) // if (!acquired) irq save
>
> For above two ", acquired = 1" as well.

I felt it would be too verbose and not accurate anyway,
since irq save will be done before the check.
It's a pseudo code.
But sure, I can add.

>
> >
> > The existing local_lock_*() macros can be used either with
> > local_lock_t or local_trylock_t.
> > With local_trylock_t they set acquired = 1 while local_unlock_*() clears it.
> >
> > In !PREEMPT_RT local_lock_irqsave(local_lock_t *) disables interrupts
> > to protect critical section, but it doesn't prevent NMI, so the fully
> > reentrant code cannot use local_lock_irqsave(local_lock_t *) for
> > exclusive access.
> >
> > The local_lock_irqsave(local_trylock_t *) helper disables interrupts
> > and sets acquired=1, so local_trylock_irqsave(local_trylock_t *) from
> > NMI attempting to acquire the same lock will return false.
> >
> > In PREEMPT_RT local_lock_irqsave() maps to preemptible spin_lock().
> > Map local_trylock_irqsave() to preemptible spin_trylock().
> > When in hard IRQ or NMI return false right away, since
> > spin_trylock() is not safe due to explicit locking in the underneath
> > rt_spin_trylock() implementation. Removing this explicit locking and
> > attempting only "trylock" is undesired due to PI implications.
> >
> > The local_trylock() without _irqsave can be used to avoid the cost of
> > disabling/enabling interrupts by only disabling preemption, so
> > local_trylock() in an interrupt attempting to acquire the same
> > lock will return false.
> >
> > Note there is no need to use local_inc for acquired variable,
> > since it's a percpu variable with strict nesting scopes.
> >
> > Acked-by: Vlastimil Babka <vbabka@suse.cz>
> > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
>
> Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>

Thanks!